Composites are combinations of two or more materials which retain their identities in the composite structure when combined to form a structural element which takes advantage of selected desirable properties of each component. The constituent materials may be organic, inorganic, or metallic and in the form of particles, rods, fibers, plates and the like. A composite structure forms an effective structure for exploiting the unique properties of certain materials such as the high tensile strength of graphite, boron, or aramid fibers.
Composite materials consist of a continuous matrix phase, which may be formed from an epoxy or polyimide, surrounding a reinforcement structure, generally formed from one of the above high strength materials. The relative roles of the matrix and the reinforcement material for a high performance composite generally assign the high strength and stiffness performance to the reinforcement material, while the matrix serves to transfer stress between materials forming the reinforcement structure and to produce a fully dense composite structure.
The fibers forming the reinforcement structure may be in the form of actual fibers, a filament or fiber of extreme length. The fibers or filaments may also be woven to produce a coherent structure. A lamina is formed by directionally binding an arrangement of fibers in a matrix. A lamina composed of fibers arranged in a uni-directional orientation and in a generally continuous roll is referred to as a "tape". A lamina composed of a cloth in the matrix is referred to as a "fabric".
A composite laminate may be formed by stacking a series of tape and/or fabric lamina and then bonding the lamina together to form a single material system. It should be noted that the lamina have directional strength characteristics from the fiber and/or cloth orientation and the lamina are stacked in a specific orientation wherein the material properties of the laminate are a function of the lamina orientation.
A structural member formed of composite materials may be fabricated using a variety of techniques. In one technique, the laminate is formed directly in the desired structural configuration by forming the lamina over a mandrel or dye in the desired configuration. The stacked lamina would then be cured by heating to a curing temperature appropriate to the matrix material, e.g., 120.degree.-175.degree. C. (250.degree. F.-350.degree. F.) for an epoxy resin matrix.
Metals may also be used to form the matrix where the fibers or cloth are embedded in the molten metal. Yet another fabrication technique uses a process known as pultrusion for forming shapes of uniform cross section, such as rods, bars, I-beams, or channels. In such a process, the matrix, which may be a thermoplastic, and the continuous fiber or cloth are formed in the desired shape while pulling on the formed product and fibers as the composite is formed in the orifice exit region of a shaped die.
All of the above fabrication techniques for composite structures provide structures having generally non-intersecting layers of fibers and/or cloth embedded in a selected matrix material. The matrix region between such parallel layers is subject to separation, known particularly as delamination, where a laminate structure is formed from lamina, and the occurrence of voids and other internal matrix defects leading to stress concentrations which can greatly reduce the strength of the composite. Although some amount of delamination and internal voids and the like can be tolerated, excessive occurrences will cause a structure to be unacceptable and rejected for use. Low production yields lead to high production costs. In addition, any design which tolerates delaminations and voids is generally overdesigned when those defects do not occur, yielding an attendant weight and size penalty in anticipation of a defect.
These and other problems in the prior art are overcome by the present invention and an improved process is provided and improved product is obtained wherein inter-lamina strength is greatly increased, stress concentrations may be more readily tolerated and structural integrity is improved.